Use of a firearm at varying distances necessitates that the shooter has knowledge and experience. To increase the shooter's performance at greater distances and in varying light conditions, optically magnified sights (henceforth referred to as “riflescope”) are used. Many riflescopes allows for the shooter to determine the range to a target via the reticle, and to adjust the point of aim with great accuracy to coincide with the point of impact; such adjustments are done in the vertical and horizontal planes through the use of adjustment knobs on top and on one or both sides of the riflescopes body (usually the right side).
To attach a riflescopes to firearms various mounting solutions called mounts or bases, and mounting devices called rings are used. In U.S. Pat. No. 2,775,817 (FIG. 4) an example of the most commonly used type of rings may be found, which are split in the horizontal plane such that the riflescope can be simply laid in place into the lower halves of the rings after which the top halves can be screwed in place against the lower halves. The problem with this solution is that the attachments between the two halves require that the rings are wide enough around the attachment areas for the threads and screws without affecting the strength of the ring. The result is that the rings obscure the riflescopes horizontal adjustment knobs, requiring the shooter to adjust his body position in order to verify or adjust the settings of the riflescope.
In U.S. Pat. No. 2,202,000 we find another common embodiment where the rings are split along the vertical plane. The problem with this solution is that the rings block the riflescopes vertical adjustment instead, resulting in the same problem as with U.S. Pat. No. 2,775,817 (FIG. 4).
In U.S. Pat. No. 3,424,420 we find a solution where a thin arcuate sheet metal saddle member is clamped against the bottom clamp members, thus replacing the upper half of the ring. The problem with this solution is that it is difficult to ensure that the riflescope retains its alignment and is not tilted to the side. In U.S. Pat. No. 2,911,723 we find a solution that is reminiscent of U.S. Pat. No. 3,424,420, but where the arcuate saddle is clamped around the tube of the riflescope and then attached to the lower half. It is known that this solution often results in surface damage to the riflescope; it also suffers from the same problem as U.S. Pat. No. 3,424,420.
Today it is very common that military and police marksmen attach optical accessories and laser marking units to their firearms as a supplement to their standard riflescope, as to provide target identification for other assisting personnel. In U.S. 2007/0199225 A1 such a sight mount is disclosed, which protrudes forward over the firearm's barrel as to allow the mounting of equipment in front of and to the sides of the ordinary riflescope. The problem with this solution is that it increases the weight of the firearm; it also raises the riflescope's position above the firearm, which requires an adjustable cheek rest as well as raising the firearms centre of gravity. Another manufacture is the McCann Industries MIRS Mount.
Another solution is to replace the top halves of rings for halves with integrated mounting solutions for accessories. Examples of such products are the LaRue STOMP and Atlantic Research Marketing Systems #22 TRC, however these solutions amplify the problem related to obscuration of the adjustment knobs of the riflescope.
It is also very common today that a shooter wants a secondary back-up sight for use at shorter ranges or in case of the primary sight getting damaged. A common position for a back-up sight is at a 45-degree angle from the standard sight, so that if the shooter wants to use the back-up sight he only has to tilt the top of the fire-arm inwards towards himself.
To ensure that any calculations or adjustments are correct, the riflescope must be mounted so that the vertical plane of adjustment coincides and is aligned with the centre-line of the firearm's bore. If the vertical adjustment is aligned with the centre-line of the bore then the horizontal adjustments will be perpendicular to the centre-line of the bore.
There are several existing technical solutions as to ensure that a firearm is held level without tilting it to the sides; in U.S. Pat. No. 5,406,733 we find a variant of a spirit level which is mounted onto a riflescope and there are several variations of this design, as well as spirit levels that are mounted onto the existing sight mount.